Laser devices generate narrow beams of light using optical amplification. Emission from lasers can be characterized as either single-longitudinal-mode or multi-longitudinal-mode emission. A key component of a laser is an optical cavity, which constrains a significant portion of the laser light to make multiple passes through a gain (amplifying) component. Laser cavities have resonant frequency modes separated by c/2 L, where c is the speed of light in vacuum and L is the optical path length of the laser cavity. These frequency modes are also referred to as longitudinal modes of the laser cavity. Single-longitudinal-mode emission generally refers to light generated by a laser when only one of these frequency modes is present. In contrast, multi-mode emission generally refers to light generated by a laser when more than one frequency mode is present at the same time. Single-mode emission is often preferred over multi-mode emission for many reasons. One important reason is that multi-mode emission creates high intensity modulation spikes (mode-beating) which can be as much as N times higher than the peak intensity of single-longitudinal-mode emission, where N is the number of longitudinal modes simultaneously present. In the case of Q-switched or short-pulse lasers, the higher peak intensity associated with mode-beating can enhance parasitic nonlinear effects such as self-focusing or stimulated Raman scattering, as the laser light is transmitted through otherwise transparent optical media. In this case, it may be acceptable that the pulses consist of different longitudinal modes, so long as only one of these longitudinal modes is present within each pulse. In some remote sensing applications, single-longitudinal-mode emission is desired to efficiently couple into a narrow absorption line or transmit through a narrow optical bandpass filter. Also, mode beating is a source of noise. An etalon, or Fabry-Perot etalon, is a device often used in laser design as a wavelength or frequency mode selecting component. An etalon typically includes two flat, parallel optical surfaces that are separated by a gap. The gap can be filled with air (as in an “air-gap etalon”) or include a solid material (as in a “solid etalon”).